Mercury -- the solar system's closest planet to the sun -- is due to be explored for the first time in almost 40 years, and this time in far greater detail. On 26 July, a U.S. spacecraft named "Messenger" is set to begin its complicated seven-year journey to Mercury, considered the least-explored of our solar system's terrestrial planets, those most like Earth.

London, 27 July 2004 (RFE/RL) -- Mercury is an inhospitable place. It is some 58 million kilometers from the sun -- almost two-thirds closer than the Earth, which is some 150 million kilometers away. The temperature on its barren, heavily cratered surface can reach 450 degrees Celsius.

Dr. Robert Massey is an astronomer at Britain's Royal Observatory at Greenwich. "Mercury is extremely hot. It is a temperature which is hot enough to melt lead during its day. But its lack of atmosphere and its very slow rotation mean the surface cools down to very, very low temperatures at night. There might actually be ice at the poles. The region never receives sunlight. If you imagine a very deep crater, the sun never reaches the floor," Massey said.

Massey says scientists know very little about Mercury. It is difficult to observe from Earth by conventional means, and the orbiting Hubble Space Telescope cannot be pointed at Mercury because of the sun's glare.

The planet, therefore, has never been properly studied, either by astronomers on Earth or by visiting space probes. "Mercury is closer to the Earth than, say, Saturn is, but it's actually quite difficult [for spacecraft] to get there. It takes a lot of energy to send a probe inwards towards the sun. And to get there, [the spacecraft is] going to make several flybys of the Earth and Venus to give it a kind of gravitational kick in the right direction," Massey said.

On 26 July, the U.S. space agency NASA plans to launch the $427 million "Messenger" mission to orbit Mercury, the first space probe to do so. The spacecraft will travel some 8 billion kilometers on its seven-year journey -- passing around Earth once, Venus twice, and three times around Mercury itself -- to work up the desired speed before entering orbit.

Massey says the difficulty of the trip is why only one spacecraft -- "Mariner 10" -- has so far visited Mercury. "Mariner 10" flew past Mercury twice in 1974 and 1975, taking pictures of only about one-half of its surface.

Doctor Louise Prockter is deputy leader of the "Messenger" imaging team at the John Hopkins University Applied Physics Laboratory in the eastern U.S. state of Maryland. "Thirty years ago, we were only able to fly by the planet, and not particularly closely. This time, we are going into orbit for a whole year, so we are going to see parts of Mercury that we've never seen. And we can study the whole environment for a long period of time. Also, we are going to be able to look at the surface in lots of different colors, so we can tell more about the composition," Prockter said.

Prockter notes that Mercury is the densest planet in our solar system and probably consists mostly of iron. And it has a global magnetic field just like Earth. So there are many things to be learned from the mission.

Even the three flybys of Mercury are expected to reveal much about the planet, which is only slightly larger than Earth's moon. The first such encounter is scheduled to take place in January 2008.

"The nice thing about the flyby is that it will be pass the hemisphere that we have not seen yet. So, even if we didn't get anything else, we would at least manage to fill in most of the gaps in our imaging," Prockter said.

She notes that one of the biggest dangers the spacecraft faces is the sun's scorching heat. It will be as though 11 suns were beating down on Earth. "We're dealing with that with a very special sunshade that's made out of ceramic cloth material, and we try and keep that between the bulk of the spacecraft and the sun at all times," Prockter said.

Prockter notes that scientists from other countries have participated in "Messenger's" construction. "We work with the BepiColombo team [eds: the European Space Agency's own Mercury probe planned for 2008]. We hope to provide them some information to help their mission to work better. And we've had some input from international collaborators. Our 'star cameras' that help us determine which way up the spacecraft is came from a company in Italy, and we've also had components from Canada, Finland, Germany and Israel," Prockter said.

"Messenger" will begin its orbit of Mercury in March 2011 -- the start of one year of intense scientific measurements. Massey of Britain's Royal Observatory looks ahead to what scientists hope the "Messenger" probe will reveal.

"I think I would say the key discovery on Mercury would be the confirmation of water ice at the poles, because if ice is ubiquitous in that way, that implies certainly that water is very, very common. And that's very good news for the big questions like whether there is life elsewhere. If you find a lot of water in the universe, it implies that the odds of finding life elsewhere are high," Massey said.

NASA calls Mercury the "last frontier" of terrestrial planets and says understanding it is fundamental to understanding the evolution of Venus, Earth, and Mars.